In video magnetic tape recorders encoded address information is often periodically recorded along with the video information on the magnetic tape. During subsequent editing of the tape, the encoded information can be read and decoded and the resulting position information used to position the tape precisely within the editing apparatus. To perform editing operations efficiently, it is desirable to have the ability to read the code signals when the tape is moving either at rewind or fast forward speeds so that the coded information can be used to roughly position the tape prior to beginning editing.
There are at least two types of code signals which are in general use at the present time. One of these code signals is a longitudinal time code signal which is recorded longitudinally on the magnetic tape in a separate audio track or cueing track which accompanies the video information.
Another type of time code signal commonly used is known as the vertical interval time code (VITC) signal. In contrast to the SMPTE time code signal, the VITC signal is embedded in the video information and recorded on the video recording track itself rather than an accompanying track. Normally, the VITC signal contains encoded information relating to the relative time at which the corresponding tape location was written and includes hour, minute, second, frame and field information and also a conventional CRC error correcting code which can be used to check for coding and reception errors.
The same VITC signal code is conventionally encoded in two non-adjacent horizontal periods within the vertical blanking interval of each video field. The coded information does not interfere with the reproduced picture because the the video signal is normally blanked during the vertical interval to allow time for the monitor scanning beam to retrace.
The VITC signal is encoded on the magnetic tape at a fixed clock or "bit" rate so that it can be decoded easily during normal editing operations when the tape is moving at normal playback speed or is stopped. It is convenient, however, to beable to decode the VITC signals at fast tape speeds in order to detect relative tape position when the tape is being run in the fast-forward or rewind mode prior to actual editing.
A problem has arisen with some prior art VITC decoding circuitry in that the VITC signal is normally decoded from the composite video signal read from the magnetic tape with circuitry that uses a local oscillator of fixed frequency. In such circuitry, the decoding oscillator is synchronized to the signals read from the tape and used to sample the incoming data stream at the normal playback bit rate. However, in some tape recording formats, the effective bit rate of the signal read from the tape varies with the tape speed over the tape recorder read head. As the tape speed increases, such as during rewind or fast forward motions, the bit rate of the composite signal read from the tape also increases. When the increased bit rate varies significantly from the normal playback bit rate, a decoding circuit which uses a fixed-frequency oscillator to sample the incoming waveform will not satisfactorily decode the signal.
Accordingly, other prior art address decoding circuits were developed which used the VITC signal to generate address information only when the tape was moving at playback speeds or was stopped. At high tape speeds the longitudinal time code was used to check position. These prior art circuits suffered from the difficulty that they required both the VITC signal and the longitudinal code to be recorded on the tape and accordingly required circuitry which could decode both types of signals.
Other prior art arrangements were developed in which the local decoding oscillator frequency is varied in proportion to the tape speed by electronically measuring the tape speed and ajusting the oscillator frequency accordingly. In these systems the tape speed is measured by detecting the time interval occurring between two fixed points on the tape such as the time interval occurring between two horizontal synchronization pulses. These latter systems suffered from additional problems in that they were subject to interference from electrical noise and the circuitry needed to check the tape speed was complex and costly.
It is an object of the present invention to derive a decoding clock rate which can be used to correctly decode the VITC signal when the tape on which the VITC signal is encoded is either stopped or running at slow or fast speeds.
It is another object of the present invention to provide apparatus in which a clock rate for decoding the VITC signal can be derived without using complex circuitry.
It is yet another object of the present invention to provide apparatus which can derive a VITC signal decoding clock rate even in the presence of noise or extraneous signals.